When blood is drawn into a blood bag, it is necessary to mix the blood with anticoagulant in order to prevent clotting. Conventionally, anticoagulant is preloaded into the blood bag before the blood is drawn. As the blood is then drawn into the bag, the bag may be periodically inverted or agitated so as to mix the incoming blood with the anticoagulant.
The task of mixing blood with anticoagulant can become more problematic if the user is unable to invert or agitate the blood bag. For example, incorporation of the blood bag into an autologous blood transfusion system may hinder the user's ability to invert the bag. One type of autologous blood transfusion system (Solco Basle Inc., Rockland MA) encloses the blood bag within a rigid evacuated chamber. Blood is then drawn or aspirated into the blood bag through a port at the top of the bag by evacuating the blood bag itself. During the blood drawing procedure, there is a tendency for the drawn blood to layer on top of anticoagulant within the blood bag. The port for evacuating the blood bag also lies near the top of the bag. Accordingly, during the blood drawing procedure, the blood bag may not be inverted so as to mix the blood with anticoagulant because, to do so, would cause blood to be drawn from the blood bag and into the vacuum line connected thereto. Clogging the vacuum line with blood is considered undesirable for a variety of reasons. Similarly, there is a risk that agitating the blood bag may cause blood to splash onto the evacuation port. Partial mixing may be achieved by gently rocking the blood bag. Such gentle rocking poses only a minimal risk of clogging the vacuum line. However, the mixing efficiency of such gentle rocking progressively declines as the blood bag approaches fullness.
An alternative arrangement for an autologous blood transfusion device is disclosed by Marx, U.S. Pat. No. 4,573,992. Marx discloses that blood may be aspirated into the bottom of an evacuated blood bag. If Marx's blood bag were preloaded with anticoagulant, mixing of such anticoagulant with drawn blood would be assured if air were periodically allowed to pass through the aspiration port. Unfortunately, it is possible for such aeration to cause blood to foam and/or denature.
What was needed was a method for automatically mixing blood with anticoagulant within a blood bag without inverting or agitating the blood bag and without causing the blood to foam.